Yet another random problem.
Tenzing has an array $a$ of length $n$ and an integer $v$.
Tenzing will perform the following operation $m$ times:
- Choose an integer
$i$such that$1 \leq i \leq n$uniformly at random. - For all
$j$such that$i \leq j \leq n$, set$a_j := a_j + v$.
Tenzing wants to know the expected value of $\prod_{i=1}^n a_i$ after performing the $m$ operations, modulo $10^9+7$.
Formally, let $M = 10^9+7$. It can be shown that the answer can be expressed as an irreducible fraction $\frac{p}{q}$, where $p$ and $q$ are integers and $q \not \equiv 0 \pmod{M}$. Output the integer equal to $p \cdot q^{-1} \bmod M$. In other words, output the integer $x$ that $0 \le x \lt M$ and $x \cdot q \equiv p \pmod{M}$.
Input
The first line of input contains three integers $n$, $m$ and $v$ ($1\leq n\leq 5000$, $1\leq m,v\leq 10^9$).
The second line of input contains $n$ integers $a_1,a_2,\ldots,a_n$ ($1\leq a_i\leq 10^9$).
Output
Output the expected value of $\prod_{i=1}^n a_i$ modulo $10^9+7$.
Note
There are three types of $a$ after performing all the $m$ operations :
$a_1=2,a_2=12$with$\frac{1}{4}$probability.
$a_1=a_2=12$with$\frac{1}{4}$probability.
$a_1=7,a_2=12$with$\frac{1}{2}$probability.
So the expected value of $a_1\cdot a_2$ is $\frac{1}{4}\cdot (24+144) + \frac{1}{2}\cdot 84=84$.
样例
2 2 5
2 2
84
5 7 9
9 9 8 2 4
975544726
题面与样例版权归 Codeforces 及出题人所有;中文翻译由 AI 生成(CI 管线),如有出入请以原文为准。